This application relates to solid state radiation detectors and in particular to phototransistors disposed in an array in an imager.
Devices used in radiation imaging for medical purposes, for example, must be able to detect incident optical photons or ionizing particles to provide a good image contrast. In such imaging devices it is often advantageous to employ radiation detection devices (e.g., photosensors) having internal gain; avalanche photodiodes (APDs) are commonly used in such devices to provide the desired detection sensitivity. In such imaging devices, it is desirable that the photosensor exhibit low noise and high gain. Certain devices, such as medical imagers (e.g., using gamma radiation), also require relatively large arrays (e.g., about 5 cm.sup.2 or larger) of high quality, low noise photosensors.
Currently, the two types of APD designs in common use are the "deep diffused" structure and the "reach-through" structure. Deep diffused APDs, however, are typically not readily manufactured in large arrays as each device must be formed to have a precise bevel at the edge of each APD in the array. The bevel is required to reduce the peak surface field (i.e., the electric field across the p-n junction in the area where the p-n junction intersects the surface of the structure) of the APD well below the peak bulk electric field (i.e., the electric field across the p-n junction in the body of the device where the p-n junction is disposed substantially parallel to the surfaces of the device to which the bias is applied) so that the APD breaks down in the bulk instead of at the surface. Further, bevel formation requires mechanical operations which make the fabrication process non-standard in that the bevel for each APD must be individually formed. The non-standard methods required for bevel formation also results in reduced yield and non-uniform reliability of the devices formed, making the fabrication of large area arrays of this type of device expensive and difficult.
The reach-through APD structure generally does not require bevel formation. The reach-through type of APD typically has a shallow p-n junction that results in lower gain, a larger value of k (resulting in high noise devices), and greater temperature drift than deep diffused devices. Further, the active area of reach-through devices is small as compared to deep diffused devices. Array fabrication can be accomplished, although the process is time consuming and expensive as many steps are required to fabricate the array, and the resulting APDs in the array suffer the drawbacks noted above. Arrays in reach-through technology are also limited to a small active area.
Conventional phototransistors are not appropriate for use in large area imaging devices because the gain in each phototransistor is relatively small with respect to APDs, the devices have a very small photosensitive area, and the devices are not readily fabricated in large area arrays. Further, the frequency response of conventional phototransistors degrades rapidly as the size of the phototransistor is increased.
For most imager devices, it is thus desirable to have a photosensor array that is readily fabricated and that contains high quality individual photosensor pixels, that is photosensors that exhibit low noise and high gain. It is also desirable that the array be structurally strong, such as a block or planar structure in which few if any cuts are needed to provided efficient operation of the respective photosensor devices.
It is an object of this invention to provide a deep-diffused planar phototransistor that is adapted to be readily fabricated in a large area array.
It is a further object of this invention to provide a large area phototransistor, e.g., having a photosensitive area in excess of 1 mm.sup.2, and typically 15 mm.sup.2 or more, that exhibits good high frequency response.